Photolithography is commonly used during the fabrication of integrated circuits on semiconductor wafers and other bulk substrates comprising a layer of semiconductor material. During photolithography, a form of radiant energy is passed through a radiation-patterning tool onto a radiation-sensitive material, commonly referred to as photoresist, which is placed upon a surface of a semiconductor wafer. The radiation-patterning tool is commonly known as a photomask or reticle.
Radiation-patterning tools contain light-restrictive regions and light-transmissive regions. Light-restrictive regions may be, for example, opaque or partially light transmissive. The light-transmissive regions or portions of a radiation-patterning tool, in conjunction with the light-restrictive regions, cooperatively facilitate the formation of a desired pattern on a semiconductor wafer. For the formation of patterns on a semiconductor wafer, the wafer is coated with a layer of radiation-sensitive material (e.g., photoresist). Subsequently, radiation passes through the radiation-patterning tool onto the layer of photoresist and transfers onto the photoresist a pattern defined by the radiation-patterning tool. Using a form of a photographic process, the photoresist is then developed to remove either the portions exposed to the radiant energy in the case of a “positive” photoresist or the unexposed portions when a “negative” photoresist is utilized. The residual photoresist pattern thereafter serves as a mask for a subsequent semiconductor fabrication process.
With advances in semiconductor integrated circuit processes, the dimensions associated with integrated circuit device features have decreased. Furthermore, the demand for smaller and faster-performing semiconductor devices requires increasing precision and accuracy in photolithographic processes.
FIG. 1 illustrates an apparatus 114 in which a radiation-patterning tool is utilized for a patterning process. Apparatus 114 comprises a radiation source 116 that generates radiation 118 and further includes a radiation-patterning tool 120 through which radiation 118 is passed. A semiconductor wafer or substrate 122 includes a radiation-sensitive layer 124 thereon. As illustrated, radiation 118 passes through radiation-patterning tool 120 and impacts radiation-sensitive layer 124 to form a pattern. This process of forming a pattern on a radiation-sensitive material with a radiation-patterning tool is commonly referred to as a printing process.
Radiation-patterning tool 120 typically includes an obscuring material that may either be an opaque (e.g., chrome) or a semi-opaque material placed over a transparent material (e.g., glass). Radiation-patterning tool 120 is illustrated in FIG. 1 as having a front side 128 for forming features or windows and an opposing back side 126. Some radiation-patterning tools further utilize both the front side and back side for the formation of windows. Radiation-patterning tool 120 typically has a pattern with dimensions on the order of, or smaller than, the wavelength of radiation passing through the radiation-patterning tool. Therefore, interference effects may occur when radiation passes through the radiation-patterning tool and exits onto the radiation-sensitive material. Accordingly, the pattern, and more specifically, geometries within the pattern of the radiation-patterning tool must be modified to compensate for such interference effects. For example, a resolution enhancement technique (RET), such as optical proximity correction may be employed to modify the pattern on the photomask or reticle to optimize the shape of the light focused on the photoresist.
FIG. 2 illustrates a flow chart of a conventional optical proximity correction process used in creating a pattern for a radiation-patterning tool. Initially, a preliminary design is created and verified 110 with a desired pattern identified to form the target pattern on the photoresist. Subsequently, profiles are developed for the radiation-patterning tool to roughly produce the desired pattern when radiation is passed through the radiation-patterning tool. The profiles or elements form a rough correspondence to the desired pattern as the profiles or elements initially disregard the effects of interference of radiation passing through the radiation-patterning tool.
Following creation and verification of the design, an optical proximity correction process 112 is used to account for various interference factors that influence radiation passing through the radiation-patterning tool. Such interference factors may include constructive and destructive interference effects resulting as the radiation wavelength approximates the dimensions of portions of the profiles or elements of the radiation-patterning tool. In an optical proximity correction process, a geometry, such as a square or a rectangle, within a pattern layout may be adjusted to compensate for optical effects (e.g., optical, micro-loading, etch, resist, etc.) such that a resultant pattern more closely approximates the desired pattern. These adjustments are made based upon results of model-based simulations of the pattern layout at so-called “evaluation points,” which are defined on the geometry to be adjusted.
As a result of the optical proximity correction process 112, a data set that corresponds to a pattern capable of generation by a radiation-patterning tool is typically generated. The data set is subsequently “taped out” to a radiation-patterning tool through the use of, for example, laser writing and/or electron-beam writing methodologies. Following the formation of the pattern on the radiation-patterning tool, the tool is capable of being utilized for semiconductor fabrication.
Conventionally, in an optical proximity correction process, the locations of the evaluation points are pre-determined through simple rules, such as by taking the mid-point of an edge of a geometry, or by considering other factors, such as the shape of the geometry. Unfortunately, these simple rules frequently do not select the optimal evaluation points for a given geometry.
There is a need for methods of enhancing a radiation-patterning process. Specifically, there is a need for methods of defining evaluation points for increased accuracy of optical proximity correction of a pattern in a radiation-patterning process.